Many wireless communication systems, such as, for example, wireless local area network (WLAN) systems, digital subscriber line (DSL) modems, digital audio broadcasting (DAB) systems, etc., employ orthogonal frequency division multiplexing (OFDM) for data transmission. The Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard addresses medium access control over a wireless local area network (WLAN). The IEEE 802.11 standard is set forth in the document IEEE Std. 802.11, entitled Supplement to IEEE Standard for Information Technology—Telecommunications and Information Exchange Between Systems—Local Metropolitan Area Networks—Specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, 1999 Edition, which is incorporated herein by reference. Additional extensions relating to the 802.11 standard, including IEEE Std. 802.11a, entitled High Speed Physical Layer in the 5 GHz Band, February 2000, and IEEE Std. 802.11g, entitled Further Higher Data Rate Extension in the 2.4 GHz Band, September 2000, also incorporated herein by reference, relate to WLAN systems which use OFDM technology in the 5 gigahertz (GHz) and 2.4 GHz frequency bands, respectively.
An OFDM system was proposed, for example, in R. W. Chang, “Synthesis of band-limited orthogonal signals for multi-channel data transmission,” Bell Systems Technical Journal, vol. 45, pp. 1775–1796, December 1966. OFDM systems provide orthogonal sub-carriers that guarantee exact reconstruction of the original data. Such systems also provide a higher degree of immunity to multi-path distortion and selective fading of channels as compared to single carrier systems. However, OFDM systems are also susceptible to errors relating to carrier frequency offset (CFO). CFO generally arises when the demodulation carrier frequency does not exactly match the modulating carrier frequency. This may result from, for example, Doppler effect or mismatched crystal frequency at the transmitter and receiver. CFO essentially destroys the orthogonality of the OFDM symbol and can cause inter-carrier interference (ICI) and inter-symbol interference (ISI).
Numerous techniques have been proposed for CFO estimation and correction in OFDM systems. See, e.g.,Dong-SeogHan, Jae-Hyun Seo and Jung-JinKim, “Fast Carrier Frequency Offset Compensation in OFDM Systems,” IEEE Transactions on Consumer Electronics, vol. 47, no. 3 (August 2001), Fu Yanzeng, Zhang Hailin, Wang Yumin, “Frequency Synchronization in OFDM Systems,” 3G Mobile Communication Technology, pp. 26–28, March 2001, Conference Publication No. 477, Yun Hee Kim, Iichko Song, Seokho Yoon and So Ryoung Park, “An Efficient Frequency Offset Estimator for OFDM Systems and its Performance Characteristics,” IEEE Transactions on Vehicular Technology, vol. 50, no. 5 (September 2001), all of which are incorporated herein by reference.
In an attempt to compensate for the undesirable effects of CFO error, the WLAN standard provides short and long training symbols to estimate CFO. Most conventional OFDM systems employ coarse and fine CFO detection and estimation using the periodic sequences sent during training. However, even a small detection error in CFO can undesirably impact the system, such as, for example, by causing a substantial reduction in signal-to-noise ratio (SNR).
There exists a need, therefore, for an improved technique that more accurately estimates CFO in a wireless communication system, without suffering from the problems exhibited by conventional wireless systems.